The present invention relates to semiconductor thin film processing by nanolayer deposition (“NLD”). The fabrication of modem semiconductor device structures has traditionally relied on plasma processing in a variety of operations such as etching and deposition. Plasma etching involves using chemically active atoms or energetic ions to remove material from a substrate. Deposition techniques employing plasma includes Chemical Vapor Deposition (“CVD”) and Physical Vapor Deposition (“PVD”) or sputtering.
PVD uses a high vacuum apparatus and generated plasma that sputters atoms or clusters of atoms toward the surface of the wafer substrates. PVD is a line-of-sight deposition process that is more difficult to achieve conformal film deposition over complex topography, such as deposition of a thin and uniform liner or barrier layer over the small trench or via of 0.13 μm or less, especially with high aspect ratio greater than 4:1.
In CVD, a gas or vapor mixture is flowed over the wafer surface at an elevated temperature. Reactions then take place at the hot surface where deposition takes place. Temperature of the wafer surface is an important factor in CVD deposition, as it depends on the chemistry of the precursor for deposition and affects the uniformity of deposition over the large wafer surface. The high temperatures typically required for CVD deposition may not be compatible with other processes in the semiconductor process. Moreover, CVD at lower temperature tends to produce low quality films in term of uniformity and impurities. More details on PVD and CVD are discussed in International Pub. Number WO 00/79019 A1 or PCT/US00/17202 to Gadgil, the content of which is incorporated by reference.
In atomic layer deposition (“ALD”), various gases are injected into a chamber for as short as 100–500 milliseconds in alternating sequences. For example, a first gas is delivered into the chamber for about 500 milliseconds and the substrate is heated, then the first gas (heat optional) is turned off. Another gas is delivered into the chamber for another 500 milliseconds (heat optional) before the gas is turned off. The next gas is delivered for about 500 milliseconds (and optionally heated) before it is turned off. This sequence is repeated until all gases have been cycled through the chamber, each gas sequence forming a highly conformal monolayer. ALD technology thus pulses gas injection and heating sequences that are between 100 and 500 milliseconds.
The ALD approach requires a high dissociation energy to break the bonds in the various precursor gases, which can be, for example, silane and oxygen. ALD thus requires high substrate temperature, for example, on the order of 600–800 degrees Celsius for silane and oxygen processes.
U.S. Pat. No. 5,916,365 to Sherman, entitled “Sequential chemical vapor deposition” provides for sequential chemical vapor deposition by employing a reactor operated at low pressure, a pump to remove excess reactants, and a line to introduce gas into the reactor through a valve. Sherman teaches exposing the part to be coated to a gaseous first reactant, including a non-semiconductor element of the thin film to be formed, wherein the first reactant adsorbs on the part to be coated. The Sherman process produces sub-monolayers as a result of adsorption. The first reactant forms a monolayer on the part to be coated (after multiple cycles), while the second reactant passes through a radical generator which partially decomposes or activates the second reactant into a gaseous radical before it impinges on the monolayer. This second reactant does not necessarily form a monolayer, but is available to react with the monolayer. A pump removes the excess second reactant and reaction products completing the process cycle. The process cycle can be repeated to grow the desired thickness of film.
U.S. Pat. No. 6,200,893 to Sneh entitled “Radical-assisted sequential CVD” discusses a method for CVD deposition on a substrate wherein radical species are used in alternate steps to depositions from a molecular precursor to treat the material deposited from the molecular precursor and to prepare the substrate surface with a reactive chemical in preparation for the next molecular precursor step. By repetitive cycles, a composite integrated film is produced. In a preferred embodiment, the depositions from the molecular precursor are metals, and the radicals in the alternate steps are used to remove ligands remaining from the metal precursor reactions, and to oxidize or form a nitride of the metal surface in subsequent layers.
In one embodiment taught by Sneh, a metal is deposited on a substrate surface in a deposition chamber by: (a) depositing a monolayer of metal on the substrate surface by flowing a molecular precursor gas or vapor bearing the metal over a surface of the substrate, the surface saturated by a first reactive species with which the precursor will react by depositing the metal and forming reaction product, leaving a metal surface covered with ligands from the metal precursor and therefore not further reactive with the precursor; (b) terminating flow of the precursor gas or vapor; (c) purging the precursor with inert gas; (d) flowing at least one radical species into the chamber and over the surface, the radical species highly reactive with the surface ligands of the metal precursor layer and eliminating the ligands as reaction product, and also saturating the surface, providing the first reactive species; and (e) repeating the steps in order until a metallic film of desired thickness is achieved.
In another aspect of the Sneh disclosure, a metal nitride is deposited on a substrate surface in a deposition chamber by: (a) depositing a monolayer of metal on the substrate surface by flowing a metal precursor gas or vapor bearing the metal over a surface of the substrate, the surface saturated by a first reactive species with which the precursor reacts by depositing the metal and forming reaction product, leaving a metal surface covered with ligands from the metal precursor and therefore not further reactive with the precursor; (b) terminating flow of the precursor gas or vapor; (c) purging the precursor with inert gas; (d) flowing a first radical species into the chamber and over the surface, the atomic species highly reactive with the surface ligands of the metal precursor layer and eliminating the ligands as reaction product and also saturating the surface; (e) flowing radical nitrogen into the chamber to combine with the metal monolayer deposited in step (a), forming a nitride of the metal; (f) flowing a third radical species into the chamber terminating the surface with the first reactive species in preparation for a next metal deposition step; and (g) repeating the steps in order until a composite film of desired thickness results.
The Sneh embodiments thus deposit monolayers, one at a time. Because the objective is to create a thick film, the Sneh process is relatively time-consuming.
Atomic layer deposition (ALD or ALCVD) is a modified CVD process that is temperature-sensitive and flux-independent. ALD is based on a self-limiting surface reaction. ALD provides a uniform deposition over complex topography and is temperature-independent, since the gases are adsorbed onto the surface. ALD can occur at lower temperature than CVD, because ALD is an adsorption regime.
As discussed in connection with the Sherman and Sneh patents, above, the ALD process includes cycles of flowing gas reactant into a chamber, adsorbing one sub-monolayer onto the wafer surface, purging the gas reactant, flowing a second gas reactant into the chamber, and reacting the second gas reactant with the first gas reactant to form a monolayer on the wafer substrate. Thick film is achieved by deposition of multiple cycles.
Precise thickness can be controlled by number of cycles, since a single monolayer is deposited as a result of each cycle. However, the conventional ALD method is a slow process to deposit films such as those around 100 angstroms in thickness. Growth rate of Atomic Layer Epitaxy (“ALE”) TiN for example was reported at 0.2 angstrom/cycle, which is typical of metal nitrides from corresponding chlorides and NH3.
The throughput in device fabrication for a conventional ALD system is slow. Even if the chamber is designed with minimal volume, the throughput is still slow due to the high number of cycles required to achieve the desired thickness. Conventional ALD is a slower process than CVD, with ALD having a rate of deposition almost 10 times slower than CVD. The process is also chemical-dependent; that is, it is necessary to ensure the proper self-limiting surface reaction for deposition.